The present invention involves the operation of an aromatic conversion unit in a manner to provide for the transalkylation of aromatic hydrocarbons in conjunction with or in reference to a toluene disproportionation reaction.
Various processes involving the disproportionation of aromatic hydrocarbons are utilized in petroleum refining operations. One commonly utilized refining process involves the disproportionation of toluene in a transalkylation reaction in which toluene is converted to benzene and xylene. The disproportionation reaction which typically takes place in the presence of molecular hydrogen supplied in addition to the toluene provides for a stoichiometric relationship in which two moles of toluene are converted to one mole of benzene and one mole of xylene. The disproportionation reaction may be carried out over a metal modified zeolite disproportionation catalyst, such as mordenite modified by the inclusion of a metal such as nickel or palladium.
Another conversion reaction employed in petroleum refining operations involves the transalkylation of polyalkyl aromatic compounds with benzene to produce a transalkylation product having a diminished content of polyalkylated aromatics with an enhanced content of monoalkylated aromatic compounds. The resulting transalkylation product exhibits correspondingly reduced benzene content. Oftentimes, transalkylation reactions are carried out in an integrated process in which an aromatic substrate, such as benzene, is alkylated with an alkylating agent, such as ethylene or propylene, to produce ethylbenzene or propylbenzene together with polyalkylated aromatics, such as dialkyl and trialkyl benzenes. The polyalkyl aromatics are separated from the monoalkyl benzene recovered from the alkylation reactor and recycled to a downstream transalkylation reactor. Benzene is also supplied to the transalkylation reactor in order to produce a monoalkylated disproportionation product, along with other alkylated aromatic compounds.
In accordance with the present invention, there is provided a disproportionation transalkylation process utilizing a metal modified zeolite disproportionation catalyst. In carrying out the invention, there is provided a catalytic reaction zone containing a metal modified disproportionation catalyst. A toluene-containing feedstock is supplied to the reaction zone which is operated under temperature and pressure conditions effective for the disproportionation of toluene. A disproportionation product containing benzene and xylene is recovered from the reaction zone. At a suitable point in the process, the supply of toluene-containing feedstock to the reaction zone is terminated and the disproportionation procedure is shut down. Thereafter, a transalkylation feedstock is supplied to the reaction zone. The transalkylation feedstock contains a benzene component and a polyalkylated aromatic component comprising at least one polyalkylated aromatic compound having at least nine carbon atoms. The reaction zone is operated under conditions effective for the transalkylation of the feedstock to produce a transalkylated product having reduced polyalkylated benzene content and an enhanced monoalkyl benzene content.
In a preferred embodiment of the invention, the disproportionation catalyst comprises mordenite. Preferably the metal modified disproportionation catalyst comprises nickel-modified mordenite having a nickel content within the range of 0.1-2.0 wt. % of the mordenite. In a further aspect of the invention, the transalkylation feedstock has a weight ratio of the benzene component to the polyalkylated benzene component within the range of 1:9-2:1. In yet a further aspect of the invention, the transalkylation product recovered from the reaction zone comprises toluene, ethylbenzene, and xylene with a content of C7 and C8 alkyl aromatic compounds within the range of 35-45 wt. %. Preferably, the ethylbenzene content of the transalkylation product is less than one-half of the toluene content, and preferably and more specifically, also less than one-half of the xylene content.
In yet a further aspect of the invention, there is provided a process for the transalkylation of an aromatic feedstock containing a benzene component and a polyalkylated aromatic component comprising at least one polyalkyl aromatic compound of at least nine carbon atoms. The reaction zone is operated under temperature and pressure conditions providing an equivalent conversion of pure toluene in the presence of the catalyst within the range of 40-55%, resulting in a transalkylated product with a reduced polyalkyl benzene content and an enhance monoalkyl benzene content relative to the transalkylation feedstock. Preferably, the reaction zone is operated under pressure and temperature conditions providing for an equivalent conversion of pure toluene in the presence of the catalyst within the range of 42-48% and, more specifically, within the range of 45-47%. In continued operation of the transalkylation reaction zone, at least one of the reaction conditions of temperature, pressure, and space velocity is adjusted in order to maintain a constant reaction severity to provide a desired equivalent conversion of toluene within a tolerance range of xc2x12%. Preferably, the temperature is progressively increased while continuing the operation of the transalkylation zone to maintain a condition of constant reaction severity.